In high-volume installations for the manufacture of frozen food products, the freezing apparatus frequently comprises a long insulated chamber, called a tunnel, through which the product passes on an endless belt conveyor. Near the outlet end of the tunnel, the food product is sprayed with a cryogen, frequently liquified nitrogen, as the final freezing step for the product. Cryogen gas, vaporized by contact with the food product and the conveyor, is directed back through the tunnel in a direction counter to the flow of the food product, progressively chilling the product as it moves through the tunnel chamber. Consequently, the conveyor belt moves continuously through zones of rapidly changing temperature, from somewhat below ambient at the entrance end of the tunnel down to temperatures as low as -320.degree. F. in the region subject to liquified cryogen spray, and back to a much higher temperature, though still very cold, at the outlet end of the tunnel.
The conveyor belt is also subject to other thermal stresses, as when the freezing tunnel is shut down for cleaning or servicing so that the belt returns to ambient temperature. Similar widely varying thermal conditions may be encountered with conveyor belts used in other thermal processing apparatus, such as heat treating tunnels or the like.
In the operation of any high speed belt conveyor system, particularly a system that is of substantial length (in a typical cryogenic freezing tunnel the conveyor length may be of the order of 60 to 80 feet) it is highly desirable to maintain a constant tension on the product-supporting run of the belt. Conventional spring tensioning devices are frequently unsatisfactory, in thermal processing equipment, due to the effect of temperature extremes on the tensioning apparatus itself. Thus, in a cryogenic freezing tunnel using conveyor tensioning springs mounted within the tunnel, the extreme low temperatures may lead to embrittlement and premature failure of the springs by thermal aging. On the other hand, if a spring tensioning arrangement is mounted outside the tunnel, it becomes difficult to maintain an adequate thermal seal. Furthermore, for long conveyor belts in a wide variety of applications, spring tensioning arrangements of adequate capacity are difficult to construct and to adjust for optimum operation of the conveyor system.
Ideally, the drive for a long belt type conveyor used in a thermal processing apparatus should provide a constant tension on the belt that automatically and inherently is adjusted to compensate for changes in the belt length due to changing thermal conditions. Moreover, the constant-tension portion of the conveyor drive should also compensate for other changes in operation of the belt conveyor, such as major variations in the operating speed of the belt or in the quantity of material supported by the belt at any given time. Compensated tension systems of this kind, to the extent known in the art, have generally not been satisfactory when applied to cryogenic freezing tunnels or other thermal processing apparatus of the kind that is normally operated within a temperature range greatly different from ambient temperature.